The present invention relates to a four-stroke cycle internal combustion engine preferably used for a power source of compact power working machine such as a portable grass trimmer, a lawn mower, a chain saw, or the like.
A portable working machine represented by a grass trimmer and a chain saw is required to allow an operator to work without any restriction on his working position or posture. Thus, an internal combustion engine as a power source mounted on such working machine is required to keep a stable operation even if it is used, for example, in a laterally tilted position.
In order to comply with such a request, conventionally, there has been generally utilized a compact air-cooled two-stroke cycle gasoline engine (hereinafter referred to as two-stroke cycle engine) which uses mixed fuel composed of fuel and lubricating oil mixed at a predetermined ratio. However, since the two-stroke cycle engine generally produces exhaust gas containing an abundance of unburned gas due to a gas-flow type scavenging system employed therein, it has a disadvantage in that it is difficult to take an effective measure to control exhaust gas or emission.
As for the emission control, the four-stroke cycle internal combustion engine (Otto engine) has the advantage of producing exhaust gas containing a small amount of unburned gas, and thereby the four-stroke cycle internal combustion engine is requested to be employed in a portable working machine in place of the; two-stroke cycle engine. In the four-stroke cycle engine, typically an oil reservoir is disposed in a bottom section of a crankcase in which a crankshaft is accommodated, and the oil in the oil reservoir is drawn by a pump or splashed by a rotating member to lubricate an inner surface of a cylinder and a valve system. However, this likely results in a complicated structure and an increased weight due to an additional mechanism for oil delivery and recovery, which is undesirable for a portable working machine.
In addition, the use of an oil reservoir forces the operator to hold the engine in a specific position (generally in an upright position) in use, which makes it difficult to apply a typical four-stroke cycle engine to a portable working machine without modification. Further, there is a fear of engine seizure due to an insufficient oil supply or lack of oil caused by a change in a position or posture thereof. Consequently, this might interfere with the operational convenience associated with the use of a portable working machine. Heretofore, various proposals have been submitted in order to improve above situation, one of which is to use mixed fuel composed of fuel and lubricant oil in the four-stroke cycle engine as in the two-stroke cycle engine.
In a four-stroke cycle internal combustion engine powered by the mixed fuel, as shown in Japanese Patent Laid-open Disclosure Nos. Hei 5-222944 and Hei 7-150920, and in Descriptions of U.S. Pat. Nos. 5,347,967 and 5,579,735, an air intake system device including a carburetor is directly connected to a crankcase defining a crank chamber, and a mist-like mixed fuel which includes lubricant oil is introduced through the air intake system device into the crankcase to lubricate the inner surface of the cylinder and the rotating members in the crank chamber, and then is introduced into a cylinder/head through an air guide tube connecting the crankcase to the cylinder head, and also lubricates the valve system in the cylinder head where it is introduced into a combustion chamber through an intake port.
Since employing a four-stroke cycle internal combustion engine using mixed fuel allows to dispense with the oil reservoir in the bottom section of the crank chamber or an independent oil supply device (a pump or a member to splash the oil) and also an oil recovery device, there are provided several advantages, for example, the stable operation of engine, that can be secured even if it is used under various positions or the postures, and that an excellent property of light weight and simple structure as the engine for the portable working machine may be maintained.
However, since the configuration for directly connecting the air intake system device including a fuel supply means such as the carburetor to the crankcase is quite different from that of the conventional four-stroke cycle internal combustion engine in which the air intake system device is connected to the intake port of the cylinder head, there occurs a problem that a great modification shall be applied to an engine design.
Additionally, since the mixed fuel to be introduced into the combustion chamber first enters the crank chamber through the air intake system device directly connected to the crankcase and then enters the cylinder head through the air guide tube connecting the crankcase to the cylinder head to go finally into the combustion chamber, the length of the air intake passage to the intake port becomes quite long, which will likely result in a problem of deteriorated startability and response.
Accordingly, an object of the present invention is to provide a four-stroke cycle internal combustion engine capable of being lubricated properly with mixed fuel containing fuel and lubricant oil without modifying a basic layout of the conventional four-stroke cycle internal combustion engine.
Another object of the present invention is, adding to the above object, to provide a four-stroke cycle internal/combustion engine adapted to control a possible deterioration of response.
In addition to the two objects described above, another object of the present invention is described above, to provide a four-stroke cycle internal combustion engine adapted to control a possible deterioration of startability.
Another object of the present invention is to provide a light weight, clean and easily operable four-stroke cycle internal combustion engine for the portable working machine.
According to the present invention, these objects of the present invention described above can be basically achieved by a four-stroke cycle internal combustion engine whose inside being lubricated using a mixed fuel composed of fuel and lubricant added thereto, the engine including:
a combustion chamber;
an intake port which opens toward said combustion chamber;
an air intake passage in communication with said intake port,
a fuel supply device for supplying said mixed fuel to said air intake passage;
a downstream side air intake passage which bifurcates from said air intake passage and located on a downstream side of said fuel supply device;
a crank chamber;
a crank shaft disposed within said crank chamber;
a branch passage for placing said downstream side air intake passage in communication with said crank chamber;
a valve chamber;
a valve system disposed within said valve chamber, said valve system including an intake valve and an exhaust valve;
a communication passage for placing said crank chamber in communication with said valve chamber;
a circulation passage for providing communication between said valve chamber and said air intake passage; and
a first valve disposed within said branch passage for opening and closing said branch passage.
According to the four-stroke cycle internal combustion engine of the present invention, since the air intake passage including the fuel supply means communicates, as a conventional four-stroke cycle internal combustion engine, with the intake port; opening to the combustion chamber to introduce an air-fuel mixture from the air intake passage through the branch passage into the crank chamber as well, the engine internals are properly lubricated by the lubricant oil contained in the air-fuel mixture. Accordingly, in the four-stroke cycle internal combustion engine of the present invention, since the oil reservoir generally used in the conventional engine is not necessary and a basic layout of the conventional four-stroke cycle internal combustion engine can be employed without modification, a practical benefit such as a reduction of development costs otherwise necessary for engine design may be provided, and the response performance equivalent to that of the conventional four-stroke cycle internal combustion engine may be secured due to the fact that the air-fuel mixture is introduced into the cylinder in a different stroke from that for the crankcase.
In a preferred embodiment of the present invention, a switch valve for opening or closing the circulation passage is interposed within the circulation passage which makes the valve chamber in communication with the downstream side air intake passage. Thus, since the circulation passage is shut off by closing the switch valve when the engine is to be started, and thereby, at least in an intake stroke, the configuration of the four-stroke cycle internal combustion engine of the present invention is substantially the same with that of the conventional one, the startability and a driving stability in the starting and succeeding period thereof can be secured.
Other objects and operational effects of the present invention will be best understood from the following description of the preferred embodiments of the present invention.